1. Field of the Invention
This invention concerns a discharge lamp used in such applications as a light source for projection equipment.
2. Description of Related Art
Overhead projectors (OHP) and liquid crystal projectors have come to be used as presentation tools in recent years. Discharge lamps, such as metal halide lamps and mercury lamps, have superior color display characteristics, and have been used as light source lamps in such projection equipment.
Discharge lamps having unprecedentedly high mercury vapor pressure, perhaps 100 atm. or more, have been developed recently in an effort to provide even greater light output and better color display characteristics. Such a high-pressure discharge lamp has remarkably high mercury vapor pressure while lit, and so the light-emitting tube and the seals formed on both sides of it require extremely great resistance to pressure.
Methods for forming the seal include the pinch seal method and the method of applying negative pressure to the bulb proper, which comprises the part that will be come the light-emitting tube and the side tubes that will become the seals, and while heating a side tube, constricting its diameter to form the seal (called the xe2x80x9cshrink seal methodxe2x80x9d hereafter). The pinch seal method is limited to a pressure resistance of about 40 atmospheres, and so the pinch seal method cannot be adopted for the discharge lamps described above; seals have been formed using the shrink seal method with its higher pressure resistance.
This shrink seal method is explained here with reference to FIGS. 4a-4c of the drawings. As shown in FIG. 4(a), a mount comprises an electrode 2 with one end bonded to a metallic foil 10 made of molybdenum 10 to 60 xcexcm thick, and an external lead bar 5, one end of which is attached to a ribbon of positioning foil 4 to allow the mount to be positioned within the bulb proper 3 and the other end of which is bonded to the opposite end of the molybdenum foil 10 from the electrode 2. Then, as shown in FIG. 4(b), the mount is inserted into the bulb proper 3 and positioned so that the positioning foil 4 is in contact with the inside wall of the side tube 3S. With negative pressure applied to the bulb proper 3, the side tube 3S is heated and shrunk to seal the side tube 3S, after which a seal is formed on the other side in the same way. Then, the unnecessary portion of the side tube 3S is cut off to complete the manufacture of a discharge lamp with shrink-sealed seals 31, as shown in FIG. 4(c).
In lamps of this sort, however, the metallic foil 10 is thin, and thus, easily bent in directions crosswise to the length of the metallic foil 10, and it sometimes happens when the mount is inserted in the bulb proper 3 and positioned that, as seen in FIG. 5 (which is a view from the direction of the edge of the metallic foil 10), the metallic foil 10 will be bent away from the center line X of the light-emitting tube, so that the electrodes 2 are shrink sealed away from the center line X.
Moreover, even if the electrode 2 is accurately positioned on the center line X of the light-emitting tube 30 prior to the shrink seal process, it sometimes happens that when the side tube 3S is shrunk during the shrink seal process, the metallic foil 10 bends in a direction crosswise to the length of the metallic foil 10, in which case the electrode 2 ends up in a position away from the center line X of the light emitting tube 30.
There has been a problem, with lamps of this sort, that the electrodes 2 would be closer than necessary to the light-emitting tube 30, and the arc would touch the tube wall and cause blackening of the light-emitting tube 30. Even if it does not come to blackening of the light-emitting tube 30, if the arc spot is not in the correct position, then if the discharge lamp is combined with a mirror, the focus of the mirror will not match the arc spot and it will not be possible to attain the expected light distribution and light output. Moreover, the voltage is decided such that the light output from the discharge lamp will stable, and if the inter-electrode distance is changed by misplacement of the electrodes 2, the voltage will change and the expected light output will not be attained.
This invention was based on the situation described above, and has the purpose of providing a discharge lamp in which the electrodes are properly positioned within the light-emitting tube.
In order to resolve the problems described above, the discharge lamp in accordance with the invention has a pair of electrodes within a light-emitting tube with an air-tight seal formed covering metallic foil attached to each electrode, in which there is a length-wise fold in the metallic foil and the electrodes are bonded to the metallic foil.